Don't rely on dart binary on PATH in run_test.py (#51302)

Author: zanderso

testing/run_tests.py

@@ -1037,10 +1037,11 @@ def run_impeller_golden_tests(build_dir: str):
                                                          ).joinpath('golden_tests_harvester')
   with tempfile.TemporaryDirectory(prefix='impeller_golden') as temp_dir:
     run_cmd([tests_path, f'--working_dir={temp_dir}'], cwd=build_dir)
+    dart_bin = os.path.join(build_dir, 'dart-sdk', 'bin', 'dart')
     golden_path = os.path.join('testing', 'impeller_golden_tests_output.txt')
     script_path = os.path.join('tools', 'dir_contents_diff', 'bin', 'dir_contents_diff.dart')
     diff_result = subprocess.run(
-        f'dart run {script_path} {golden_path} {temp_dir}',
+        f'{dart_bin} run {script_path} {golden_path} {temp_dir}',
         check=False,
         shell=True,
         stdout=subprocess.PIPE,
@@ -1052,9 +1053,8 @@ def run_impeller_golden_tests(build_dir: str):
       raise RuntimeError('impeller_golden_tests diff failure')
 
     with DirectoryChange(harvester_path):
-      run_cmd(['dart', 'pub', 'get'])
       bin_path = Path('.').joinpath('bin').joinpath('golden_tests_harvester.dart')
-      run_cmd(['dart', 'run', str(bin_path), temp_dir])
+      run_cmd([dart_bin, 'run', str(bin_path), temp_dir])
 
 
 def main():

tools/dir_contents_diff/test/dir_contents_diff_test.dart

@@ -1,3 +1,7 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'dart:io' as io;
 
 import 'package:litetest/litetest.dart';
@@ -19,11 +23,10 @@ void main() {
     return;
   }
 
-
   // Runs `../bin/dir_contents_diff.dart` with the given arguments.
   (int, String) runSync(String goldenPath, String dirPath) {
     final io.ProcessResult result = io.Process.runSync(
-      'dart',
+      io.Platform.resolvedExecutable,
       <String>[binPath, goldenPath, dirPath],
     );
     return (result.exitCode, result.stdout ?? result.stderr);